X-ray scattering may be employed for inspection of personnel, vehicles, cargo, or other objects of interest. The term “object” is used inclusively herein to encompass any of the above. In systems employing x-ray scattering, x-rays are formed into a beam that is directed towards the object of interest. When the beam hits the object, scattered X-rays are captured by x-ray detectors and various characteristics of the scattering object may be ascertained, either globally, or with respect to a pixelated image of the object.
The resolution of information obtained about the interrogated object or person is dependent upon a variety of factors including the distance between the inspection system and the object, and the magnitude and energy spectrum of the x-ray flux. In current systems, as the distance between the X-ray system and the object increases or as the flux decreases, the image resolution and quality (as manifest in the signal-to-noise, for example) decreases. The decrease in quality is substantially caused by the reduction of backscattered flux captured by the detectors. Current backscatter x-ray imaging systems locate detectors adjacent to the x-ray source, allowing the combined system of source and detectors to be as close as possible to the object being inspected. The proximity of the system to the object creates a high quality image without the need for a high x-ray flux.
However, there are many applications, especially security and surveillance applications, where a larger distance between the imaging system and the object to be inspected would be desirable. One such application is where personnel to be inspected might be carrying explosive devices carried under clothing or concealed in backpacks or bags and the risk of suicide detonation is present. Suicide bombings have often entailed large quantities of metal shrapnel packed around the explosive to maximize the lethality of the device, typically nuts, nails, or ball-bearings.
Current x-ray inspection systems are often inadequate in such applications and are rarely used in applications requiring distances greater than five feet. Current systems can counteract the decrease in image quality by increasing the size of the detectors or using higher flux x-ray sources. However, if the distances are too great, the detectors required will be impractically large. Additionally, as the flux increases, so will the objects exposure, which poses a problem when the object is, or may contain, a person.
One scenario for backscatter inspection from a mobile inspection vehicle is described in U.S. Pat. No. 7,099,434, to Adams et al., issued Aug. 29, 2006 and incorporated herein by reference. Embodiments of that invention can be highly effective at detecting large quantities of explosives or other organic materials in vehicles or other containers. One consideration, however, is that metal objects (such as artillery shells) within a metallic container (such as a vehicle) may not be well-detected unless favorably silhouetted against a brightly scattering background of organic material.
Another issue for backscatter technology is that it can sometimes be difficult to image organic materials when they are placed within or behind significant amounts of high-Z material, such as steel. An example of this might be a small quantity of explosive concealed in the trunk of a vehicle. Because the backscattered x-rays are typically detected in the backward direction (scatter angles typically in the range 140°<⊖<180°), the average energy of the scattered x-rays is quite low (about 68 keV for a primary x-ray beam from a 225 kV x-ray source). These low-energy x-rays are then greatly attenuated by the steel body of the vehicle, resulting in a greatly reduced number of scattered x-rays being detected in the backscatter detectors. This problem is often exacerbated because the scattered x-rays reach backscatter detectors having passed through an intervening steel surface at an oblique angle, resulting in an effective thickness of steel that is greater than the actual gauge of the steel.